The invention relates to a process for heating the surface of a substrate using a hot gas jet, particularly employing simultaneous feed of a substance for carrying out surface treatment or coating in accordance with the flame spraying process, in which the combustion gas to be mixed with the combustion air and which is supplied in an annular fashion is accelerated towards the surface to be heated by introducing compressed air in the form of a pumping jet.
The invention further provides a burner for heating the surface of a substrate, particularly in combination with a nozzle for the substance used for the surface treatment or coating and used in the flame spraying process consisting of a preferably coaxial nozzle for compressed air having an axial flow component and an annular guide plate surrounding the nozzle and spaced therefrom defining an annular channel having openings at its rear for the introduction of air and further comprising a concentrically arranged combustion gas nozzle.
In the case of a known burner of this type, the compressed air supplied through the axial nozzle simultaneously transports the coating substance in particulate form. Using the injector principle, the pumping jet of compressed air sucks in air from the outside via the annular channel which is open at the back. The air which is drawn in becomes mixed, in the annular channel, with the combustion gas which is fed in by means of an annular nozzle located at the outer edge of the annular channel, so that a crown-shaped flame results which surrounds, in the form of a mantle, the central cone-shaped compressed air jet loaded with the particles of coating substance, so that between this mantle and the cone-shaped compressed air jet, a jacket made up by the air which has been drawn in is formed. The hot gas flame mantle heats the surface to be coated and dries it. Additionally, it shields the cone-shaped compressed air jet from external influences and heats it using radiation and turbulence along its path from the outlet from the nozzle up to the surface to be coated.
The advantages of a method carried out using a burner of this type consist in the fact that coating occurs in a region where the moisture content of the air is extremely low. The necessary heat energy required for drying the surface can be produced sufficiently rapidly and carried along without the temperature along the transport path becoming too high. It is even possible to add flammable and low boiling point solvents to the coating substance without them becoming ignited during the spraying process. These advantages justify the succesful use of the known process and the known burner as the flame spraying gun; nevertheless, the burner is not devoid of disadvantages. Thus, pulsation of the flow is found to occur which leads to incomplete combustion of the combustion gases. Pulsation also leads to uneven heating of the surface, since the flame then tends to oscillate. When the pulsation becomes too great it can even bring about extinction of the flame.